The present invention relates to synchronization signal detection in video decoders.
The satisfactory reproduction of a picture requires the transmission of several types of information combined into a single waveform called a composite video signal. The signal is composed of video information and synchronizing information. Composite video describes a signal in which luminance, chrominance, and synchronization information are multiplexed in the frequency, time, and amplitude domain for a single-wire distribution. Luminance is defined as the component signal in color video systems that represents the brightness of the image. Chrominance is defined as the component signal in color video systems that describe color-difference information (and can largely be ignored for the purposes of this application).
The video signal conveys information concerning the blanking level, the black reference level, average scene brightness level, picture details, and color values. The baseband video signal is unipolar with one direct current (xe2x80x9cDCxe2x80x9d) level (nominally 0 volts) representing black, and a second level (nominally +700 mV) representing white. Any level between 0 and 700 mV represents a degree of gray.
The synchronizing information consists of horizontal and vertical scanning synchronization, and chrominance decoder synchronization. The horizontal and vertical synchronization information is used to trigger the horizontal and vertical deflection circuits in the receiver. The horizontal sync tells the display where to put the video signal in the left-right dimension, and the vertical sync tells the display where to put the signal in the top-bottom dimension. Synchronization consists of pulses having a specific amplitude, duration, and shape best suited to the task at hand. The synchronizing pulses are unipolar with a reference level of 0 V and a peak negative level of nominally xe2x88x92300 mV.
The video signal waveform, with a nominal peak-to-peak amplitude of 700 mV, and the synchronizing signal waveform with a nominal peak-to-peak amplitude of 300 mV, are added together to form a composite video signal of 1 V peak-to-peak. The synchronizing pulses are placed in parts of the composite signal that do not contain active picture information. These parts are blanked (forced below a black level) to render invisible the retrace of scanning beams on a correctly adjusted display.
The standard video signal levels apply to both conventional television scanning standardsxe2x80x94National Television System Committee (xe2x80x9cNTSCxe2x80x9d) and Phase Alternating Line (xe2x80x9cPALxe2x80x9d). The U.S standard is NTSC which uses 525 lines at 60 Hz, while PAL is predominant in Europe and uses 625 lines at 50 Hz. Composite video signals are expressed in IRE units. An IRE unit is defined as one-hundredth of the excursion from the blanking level (0 IRE units) to the reference white level (100 IRE units). A standard 1 V peak-to-peak signal is said to have an amplitude of 140 IRE units of which 100 IRE units are luminance, and 40 IRE units are synchronization information. Further discussion of video circuits and signals can be found in the following texts: M. Robin, DIGITAL TELEVISION FUNDAMENTALS, McGraw-Hill (1998); K. Jack, VIDEO DEMYSTIFIED, 2nd Edition, Harris Semiconductor (1996); and A. Inglis, VIDEO ENGINEERING, 2nd Edition, McGraw-Hill (1996), all of which are hereby incorporated by reference.
A frame of video is essentially one picture or xe2x80x9cstillxe2x80x9d out of a video stream of pictures. In NTSC, a frame comprises 525 individual scan lines (for PAL 625 lines). For NTSC, after 525 lines have been displayed on the screen, the picture presentation process continues with the next frame of 525 lines. An interlaced TV screen (and only an interlaced scanning system) is made using two fields, each one containing half of the scan lines needed to make one frame. Although in analog terminology, each field is considered to have 262.5 lines, in the digital domain, it is convenient to consider each field comprising a whole number of lines; 263 for the odd field, and 262 for the even field. For NTSC, the lines number 1-263 for the odd field, and 264-525 for the even field. The composite video signal contains a vertical sync pulse which signals the start of the odd and even fields. The first 9 lines of both the odd and even fields are vertical sync pulses. Each field is displayed in its entiretyxe2x80x94therefore, the odd field is displayed, then the even field, then the odd field, and so on. The vertical scan frequency is chosen so that half of the scanning lines are contained in each field. This causes the first line of alternate fields to begin in the center of the picture, and the lines are interleaved between fields. Each field occurs at a rate of 60 Hz for NTSC (50 Hz for PAL) television standards.
Non-standard video sources present problems to logic that is designed for standard video inputs. In a non-standard video mode, video signals obtained from sources such as the VCR in rewind, fast forward, pause modes, and video games, may output frames which do not have the standard number of lines (e.g. 528 lines in a 525-line NTSC standard). A non-standard signal exhibits a lack of serration pulses which normally indicate the start of the sync pulse. (Vertical sync is identified by broad pulses, which are xe2x80x9cserratedxe2x80x9d in order for a receiver to maintain horizontal sync, even during the vertical interval.) The signal may instead provide one large broad pulse where the serration pulses are normally expected in a standard video signal. The absence of sync level between the end of a broad pulse, and the start of the following sync pulse is called serration. Thus decode from the line counter is not possible. The inability to detect a non-standard signal may result in vertical roll of the picture presented to the viewer. This non-standard mode must be detected and a vertical sync output when a sync is detected at the input.
Another problem arises when there is no video input present. Here it is still desirable to output a vertical sync in a free running mode so that a blank screen is displayed on the monitor. Automatic detection of these three modes is a desired feature.
The present application discloses a technique for automatic detection of three modes of input video signals. Furthermore, upon detection of a particular mode, the circuit automatically adapts to the input signal, and outputs a vertical sync pulse based on the input sync signal, or lack thereof. In a first mode, a standard video signal is received and a line counter is used to decode and output the vertical sync. In a second mode where a non-standard signal is received, the line counter cannot be used, but a correct vertical sync is generated by a special procedure. In a third mode, no video input signal is received, yet a vertical sync is output in free-running mode so that a blank screen is displayed. The video decoder must recognize the three modes of video input, detect the vertical sync pulse, and then output a vertical sync pulse for each of the two fields. For a standard video mode, a vertical line counter (clocked and incremented by the horizontal sync pulse at the input) controls the modulus of the line counter, and is used to decode and output a vertical sync pulse. Odd and even vertical sync pulses may be determined by the relationship between the detected vertical sync and the horizontal sync at the input. It counts from 1 to a terminal count based on the video standard employed (525 for NTSC and 625 for PAL). A value of 1 indicates the start of vertical sync for an odd field and a value of terminal count divided by 2 indicates the start for an even field. The particular video standard (NTSC or PAL) is used to set the counter modulus.
An advantage of the disclosed method is automatic detection of and adaptation to NTSC and PAL standards based on horizontal line count. Another advantage is detection of a non-standard mode of operation used for non-standard video input (VCR, video games, etc.) where vertical sync output is generated when detected at the input. Another advantage is a start-up mode of operation used for power-up, reset, and no video input. Another advantage is the use of even and odd windows to enable sync detection, and filter out undesirable noise. Another advantage is a microprocessor controlled means of vertical sync detection and output generation; control parameters may be easily adjusted for optimization in various applications. Another advantage is the use of half-line pixel accumulators to detect vertical sync in the input signal and filter out undesirable noise. Another advantage is the threshold for sync detection adapts to varying input signal conditions. Another advantage is three modes of operation for ease and flexibility for different types of video inputs. The algorithm automatically switches between the different modes based on input signal characteristics such as lines per frame, and no sync, standard sync, or non-standard sync detection. Another advantage is that the standard mode of operation is used for standard video where vertical sync output is generated based on the horizontal line count. A flywheel effect is implemented which enables continuous output even when momentary loss of sync at the input occurs for several frames.